1.1 Technical Field
The invention relates to mobile communication devices. More particularly, the invention relates to base station soft handoff in IP-based CDMA Networks.
1.2 Related Art
Handoff is a process in which a mobile station communicating with one base station (referred to as the serving base station or SBS) is switched to another base station (referred to as the target base station TBS) during a call. Soft handoff is a form of handoff in which a mobile station starts communicating with the target base stations without interrupting the communication with the serving base station. Soft handoff has been shown to be an effective way for increasing the capability, reliability, and coverage range of CDMA networks. Soft handoff also helps reduce outage area size, achieves macro diversity gain, and provides more time for carrying out the handoff procedure.
Soft handoff may be implemented in an IP-based wireless network as illustrated in FIG. 1. The coverage area of the wireless network consists of cells 102, 103, and 104. Each cell is a geographical area that has a single wireless base station BS1, BS2, and BS3, respectively. The wireless network includes an IP network backbone 101 as is know in the art. An example of an IP-based wireless network is CDMA.
Soft handoff may be defined as a mobile station or mobile terminal receiving the same data from two or more base stations at the same time. Also, the mobile station or mobile terminal next combines the copies of the data from the different base stations into a single data stream and regenerates packets for applications associated with the mobile terminal. Soft handoff regions of FIG. 1 are represented by overlapping regions of cells 102, 103, and 104.
FIG. 2 illustrates how conventional centralized wireless networks implement soft handoff. FIG. 2 shows a serving base station 204 and a target base station 205 both communicating with a mobile terminal MS 206. A serving base station 204 is a base station that is acting as a primary base station for the mobile terminal 206. The target base station 205 is a base station that has recently become known to mobile terminal 206 or whose signal power (or other signal characteristic, e.g., signal to noise ratio) is above a threshold. A selection and distribution unit 201 generates packets 202 and 203 and sends them to serving base station 204 and target base station 205, respectively.
Three activities occur in soft handoff including transmission frame synchronization, data content synchronization, and data content combination.
With transmission frame synchronization, the mobile station collaborates with the base stations to synchronize the radio channel frames received (and transmitted) by the mobile station.
With data content synchronization, the centralized Selection and Distribution Unit (SDU) is responsible for distributing traffic, over layer-2 circuits, via different base stations to the mobile station and ensuring that the matching link-layer (and physical-layer) frames sent to different base stations contain copies of the same data. This applies to the forward direction (from the SDU 201 to the mobile terminal 206). In the reserve direction, the mobile terminal 206 ensures that the matching link-layer frames sent to different base stations contain copies of the same data.
With data content combination, in the forward direction, the mobile station combines the radio signals received in the matching frames from different base stations to generate a single final copy of each piece of received data. In the reverse direction, the SDU 201 selects one of the data received from different base stations.
To support the growing number of users of IP networks, base stations are believed to need to become autonomous. However, the current approaches for supporting soft handoff in an IP wireless network do not readily support autonomous IP-based base stations. These limitations include: the inability of mobile terminals to identify copies of the same data due to IP address changes, loss of data content synchronization, the inability to combine duplicate IP packets due to multiple link-layer connections to the mobile station, and the conflict with network layer or above layer mobility support.
First, mobile stations served by different base stations may have to use disjoint sets of IP addresses. This, for example, is likely the case when the mobile stations are served by different base stations that belong to different IP subnets. In such cases, the approach designated in FIG. 2 over, for example, WCDMA networks will cause packets sent from different base stations to the same mobile station to carry different destination IP addresses. Consequently, the mobile station 206's radio system, which operates at protocol layers below the IP layer, will not be able to determine whether two pieces of data, each arriving from a different base station, are copies of the same data. This frustrates the soft handoff requirements of data content synchronization, and data content combination as defined above. Accordingly, mobile stations may experience difficulty in identifying copies of the same data due to IP address changes.
Second, in conventional centralized CDMA networks, data content synchronization is achieved using a centralized control entity—the Selection and Distribution Unit 201. In particular, the SDU 201 ensues that data contents carried in matching frames 202 and 203 sent to different base stations are copies of the same data. In autonomous IP-based wireless base stations, centralized control entities will no longer exist. Consequently, even though the CDMA radio system is capable of synchronizing the link and physical layer frames on the radio channel, it cannot guarantee that the matching frames from different base stations will carry copies of the same data. This is because the copies of the same IP packet may arrive at different base stations at different times and may be transmitted by the base stations to the mobile station at different times. As a result, the mobile station's radio system may not be able to determine which frames received from different base stations contain copies of the same data. This problem is illustrated in FIG. 3.
FIG. 3 shows frames 301-305 from a serving base station and frames 306-310 from a target base station. The mobile terminal 206 receives frame B 301 from the serving base station as frame 1. Due to propagation delay or network delay or other random delays, frame B 309 is not received from the target base station until frame 4, suffering from a random delay 311. Accordingly, when the mobile terminal attempts to combine the first frame received, the mobile terminal will attempt to combine content B from the serving base station with the content X from the target base station. This disruption in the content from the serving base station arriving at the same time as the content from the target base station may be referred to as the loss of data content synchronization.
Different types of synchronizations between the source base station and the target base stations may be used. The types of synchronization include time-stamping the transmissions, transmitting the signals during a predefined window and attempting to synchronize the received transmissions based on pattern matching the received packets.
Third, transporting IP packets over CDMA networks (or over any other network) requires a link layer protocol that is capable of packet encapsulation/framing, i.e., the ability to reassemble IP packets at the receiving host. Today, for example, most proposals for IP-based CDMA networks use the Point-to-Point Protocol (PPP) defined by the Internet Engineering Task Force (IETF) for this purpose (see D. Perkins, “The Point-to-Point Protocol: A Proposal for Multi-Protocol Transmission of Datagrams Over Point-to-Point Links” IETF RFC 1134, November 1989.). However, link layer protocols like PPP could, without further innovations, make soft handoff impossible in many cases. For example, a single PPP connection, even when the PPP Multilink Protocol is used, cannot support multiple simultaneous copies of the same traffic stream between a base station and a mobile. Multi-link PPP can be modified to transport multiple copies of the same data simultaneously via different base stations to the mobile station. However, this cannot solve the data content synchronization problem. If a separate PPP connection is used by the mobile station to connect to each different base station, the conventional standard link-layer protocols will not be able to combine the packets arriving at the mobile station from different PPP connections. Consequently, duplicate IP packets have to be delivered to the IP layer. Unfortunately, the conventional standard IP layer protocols cannot combine duplicate IP packets either. As a result, the soft handoff requirement of data content combination as described above is violated.
The transmission control protocol (TCP) (e.g., J. B. Postel, Editor, “Transmission Control Protocol” IETF RFC793, September 1981), which is immediately above the IP layer, can detect and discard duplicate IP packets. However, TCP does not combine IP packets. The user datagram protocol (UDP) (e.g., the UDP (User Datagram Protocol)(see J. B. Postel, “User Datagram Protocol”, IETF RFC768, August 1980), which is also immediately above the IP layer, does not even detect duplicate IP packets; it simply passes on each incoming IP packet to the upper layer applications. Consequently, soft handoff cannot be achieved due to the inability to combine duplicate IP packets. This may be understood as an inability to combine duplicate IP packets due to multiple link-layer connections to the mobile.
Fourth, sometimes soft handoff involves IP address change of the mobile station. For instance, if the target base station belongs to the different IP sub-network from the serving base station and starts to take the “serving” role because the mobile station is approaching enough near to the target base station, the IP packets to the Mobile Station have to be directly routed to the target base station that now becomes new serving base station. The IP address of mobile station has to be changed to the new one belonging to the same sub-network address range as that of the target base station. This relates to problems in data content combination if PPP is used, since this makes the migration of the PPP end point on the base station side. Usually this IP address change is managed by the mobility management function in the network layer and works independently from a layer 2 system. In this regard, an unexpected change of the serving base station in link layer breaks IP communication over the link. Here, the network layer mobility management recovers the connection and an expected packet passing route change by the network mobility management breaks the soft handoff processed in the layer 2. This happens when the base station works autonomously and there is no layer 2 anchor point like SDU any more. Instead of such disruption, seamless and smooth handoff for the IP communication is required even in this case.